The recA protein of E. coli, which mediates DNA strand exchange reactions, has been extensively studied biochemically for the insight it may provide into general genetic recombination. It is hoped that a process as complex as the eucaryotic meiosis may be elucidated through the use of in vitro recombination systems which are based upon the enzymatic action of the recA protein. Structural studies of the recA protein will also be important in understanding how a single protein can mediate homologous recognition, perform strand exchange, dramatically alter the twist and axial rise of DNA bases, act as a protease, and regulate its own polymerization, among other activities. Electron microscopic studies, image analysis, and x-ray fiber diffraction have the potential for bridging the gap between crystallographically determined structures and atomic models on the one hand, and solution biochemistry on the other. Electron microscopic studies have shown that the recA polymer can be seen in several different conformational states, and these states are almost certainly related to steps in the recombination process. A research plan based upon image analysis of different states of the recA polymer is proposed. Specifically, it is proposed that three-dimensional reconstructions of the recA polymer will be computed from polymers prepared under different conditions. These are the self-polymer, complexes of recA with single-stranded DNA and complexes with double-stranded DNA, with the polymers being examined with different nucleotide cofactors. Complementary studies on the structure of the polymers will be done with x-ray fiber diffraction. Preliminary results of image analysis have shown: 1) at least two very different conformations of the recA polymer can be reconstructed; 2) information about the stoichiometry between recA and DNA bases can be determined; 3) image analysis can illuminate the deformability of the recA polymer, which may be important to its function.